Insulated electric conductor



May 7, 1935- E. c. PITMAN 2,000,095

INSULATED ELECTRIC CONDUCTOR Filed March 11, 1931 EarleCDflman, INVENTOR BY ms ATTORNEY Patented May 7, 1935 UNITED STATES PATENT OFFICE du Pont do Ncmourl a M, a corporation of Delaware Application March 11, 1981, Serial No. 521,629 7 Claims. (Cl. 178-284) This invention relates to insulated electric con-, ductors and more particularly to oil resistant conductors of this type.

The ordinary insulated electric conductor comprises wire surrounded by an insulating layer of rubber and an outer fabric layer, usually of braided cotton, silk, asbestos, rayon, or the like. Such conductors have been coated with various varnish compositions to give the wire further protection, and more recently, cellulose ester lacquers have supplanted varnishes for this purpose as they dry faster, give a better appearance and are much more resistant to oil and grease which ruin the insulation if they come in contact with the rubber layer.

Although the cellulose ester lacquers are highly resistant to oil and grease and are in general satisfactory for this purpose, a conductor coated with such lacquers is not entirely satisfactory because the lacquer does not appreciably impregnate the fabric layer and this layer, if exposed to an oily or greasy substance at any point, will absorb it, acting like a wick and bring it in contact with the rubber layer. The rubber, upon exposure to the oil or grease, is partially dissolved and swells thereby disrupting the lacquer coating and ruining the insulation. The lacquer coating does not completely prevent the fabric layer from r contacting with oil or grease in any case because the fabric layer is exposed where the conductor is cut for connection to terminals. Also the coating is quite apt to be accidentally cut by a sharp instrument in handling the conductor, thereby exposing the fabric layer. Frequently the fabric layer is imperfect, having a fuzz or stray fibers which stick up and are not sealed by the coating and such imperfections also present a point where oil may get through the coating and be absorbed by the fabric layer.

The wick like action of the fabric is extremely great and oil will be absorbed to a distance of three or four inches along the conductor on both sides of the slightest break in the coating, which largely deitroys the insulating properties of the protective layers around the wire.

An object of this invention is to provide an insulated electric conductor which is resistant to oil and grease and thereby overcome the above described objections to the conductors now known. A further object is to provide an insulated conductor that is resistant to oil and grease even though an outer coating of lacquer or varnish is not used.

These objects are accomplished according to this invention by impregnating the fabric layer,

or the individual strands from which the fabric layer is to be made, surrounding the layer of .rubber insulation of an insulated electric conductor with. an oil resisting composition and thereafter, if desired, applying a lacquer or varnish coating to the conductor.

In the accompanying drawing, Fig. l, is shown a longitudinal section of an insulated conductor illustrating one preferred embodiment of the present invention and Fig. 2 is a transverse section on the line 22 of Fig. 1.

The conductor comprises a wire i surrounded by a layer oi! rubber 2 which in turn is surrounded by a layer of fabric 3 which may be braided or not, as desired. This fabric layer 3 is impregnated with an oil resisting composition I and thereafter a lacquer or varnish coating 4 is applied to the conductor.

The preferred impregnating medium is an air drying resin solution. Any resin which will retain its flexibility on ageing and exposure to the ordinary conditions to which insulated conductors are subjected and which is resistant to oil and grease may be used. If the conductor is to be subsequently coated with lacquer or varnish, a resin that will not adversely affect the subsequent coatings should be selected. Resins formed from polybasic acids and polyhydric alcohols, such as diethylene glycol phthalate, are particularly suitable, as are the polymerized vinyl compounds, such as polyvinyl acetate. Natural resins which are resistant to oils, such as shellac, are adapted for this purpose.

Modifiers may be added to the impregnating medium to increase the flexibility of the finished product. Some resins, such as diethylene glycol phthalate, are suiliciently flexible by themselves on ageing so that the addition of a modifier is not essential but, on the other hand, shellac, and many other resins are considerably improved in flexibility by the addition of modifiers.

Such

kept low.

The ordinary resin solvents may be used,

but

it is an advantage to use the ester and alcohol solvents rather than the hydrocarbons if the conductor is to be later coated with a cellulose ester lacquer because the hydrocarbons dissolve the paraflin usually present on the surface of the rubber insulating layer and this tends to prevent the lacquer coat from drying.

The impregnation is conveniently carried out by passing the conductor, before any varnish or lacquer coat has been applied, through a bath of the impregnating medium. However, the strands from which the fabric layer is to be made may be impregnated before being braided. The lacquer or varnish coat is subsequently applied in the usual manner, preferably by passing through a bath of the lacquer or varnish, and shows an excellent adhesion to the resin impregnated fabric. It is not necessary that the impregnated fabric be completely dry before coating with the lacquer or varnish.

The following example is given to illustrate the advantages of an insulated electric cable made according to the present invention over those previously used:

Two sections of 7 mm. ignition cable, comprising a. wire coated with a layer of rubber insulation and surrounded by a layer of cotton braid, were cut from the same cable and treated as follows: One section was dipped in a 50% solution of diethylene glycol phthalate in ethyl acetate and allowed to dry for about 15 minutes, at the end of which period the solvent had evaporated. It was then coated with a clear cable lacquer of the following composition:-

Parts by weight The other section of the cable was coated directly with the same cable lacquer used above, omitting the impregnation with the diethylene glycol phthalate solution.

A small pin prick was made in the lacquer coating of each section at about the middle of the section and both were then immersed in light lubricating oil for 24 hours. Upon examination after this immersion, the section coated with the lacquer alone had. absorbed oil and carried it by the capillary or wick action of the fabric to a distance of about three inches on each side of the pin prick. The oil, having been brought into contact with the rubber layer, caused it to swell and become brittle. swelling of the rubber would have continued to a point where it disrupted the lacquer coating entirely which would have caused a complete breakdown of the insulation. On the other hand, the section which had been impregnated first with the diethylene glycol phthalate solution showed absolutely no oil penetration.

When the fabric of these sections was removed from the insulation it could be readily seen that where the lacquer had been used alone it had not penetrated into the fibers of the fabric and the fabric accordingly presented a dry, fuzzy, wick-like appearance and, as the test showed, rapidly absorbed oil. The section which had been first impregnated showed that the diethylene glycol phthalate solution had completely penetrated through the fabric and that this satura- In due course, the,

tion of the fabric prevented any penetration of the 01].

Although the above test was applied to a cable having a layer of cotton fabric, the invention is equally applicable to rubber insulated cable having silk, asbestos, rayon, or similar fabric layers.

Other sections of insulated electric conductors having a rubber layer surrounding the wire and an outer fabric layer were impregnated with the following resin solutions:

pregnated was immersed for a week in oil with- ,out any indication of the oil being absorbed by the fabric layer.

It is not necessary that the conductor be coated with a lacquer or varnish after the impregnation, but it is usually desirable to apply a. lacquer to the cable as it gives the finished cable a better appearance and greater resistance to abrasion and also oil.

The particular type of cellulose ester lacquer used, or varnish, if it is desired to use a varnish, is immaterial and any type of commonly used lacquer or varnish is suitable for this purpose, although it is advantageous to use a cellulose ester lacquer having high flexibility and oil resistance.

The invention has been described particularly with respect to electric conductors having 'a fabric layer surrounding the rubber insulation. It will be understood by those skilled in the art that a layer of any fibrous material which has a wicking action can be substituted for the fabric layer without departing from the spirit of this invention.

As many apparently and widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

I claim:

1. An electric conductor comprising a wire, a layer of rubber surrounding the wire, a fabric layer surrounding the rubber layer and an outer cellulose lacquer coating, said fabric layer being impregnated with a solution of a synthetic resin resistant to oil.

2. An electric conductor comprising a wire, a layer of rubber surrounding the wire, a layer of fabric surrounding the rubber layer, and an outer cellulose lacquer coating, said fabric layer being impregnated with a polyhydric alcohol-polybasic acid type of resin.

3. An electric conductor comprising a wire, a layer of rubber surrounding the wire, a fabric layer surrounding the rubber layer, and an outer cellulose ester lacquer coating, said fabric layer being impregnated with diethylene glycol phthalate.

4. An electric conductor comprising a wire, a layer of rubber surrounding the wire. a fabric layer surrounding the rubber layer and an outer cellulose ester lacquer coating, said fabric layer being impregnated with polyvinyl acetate.

5. An electric conductor comprising a wire, a layer of rubber surrounding the wire, a fabric layer surrounding the rubber layer, said fabric layer being impregnated with a composition comprisingpm a resin resistant to oil and tricresyl phos- 6. An electric conductor comprising a wire, a layerofrubbersurrmmdingthewiraafabric layer surrounding the rubber layer, and an outer cellulose ester lacquer coating, said fabric layer being impregnated with a composition comprising a resin resistant to oil and a modifier to increase the flexibility of said resin.

7. An electric conductor comprising a wire, a layer of rubber surrounding the wire, a fabric layer surrounding the rubber layer, and an outer cellulose ester lacquer coating, said fabric layer being impregnated with a composition comprising 10 a resin resistant to oil and tricresyl phosphate.

EARL-E C. PITMAN. 

